Hmm, I think this just might be a situation of our own making. When we assign a title to a person, we draw imaginariy mental lines and we hold an implicit bias that says only they are capable of doing that thing, rather than, it's their primary goal to be doing that thing. Similarly, others outside that title will hold a prejudice thinking, what is the big deal with what they are doing, what's taking so long, couldn't someone else do it?

The question can easily be shifted to ask, why only devs doing development, couldn't PMs and designers do it too? Well, yes they can. At least parts of it. HTML, CSS, definitely, maybe some basic JS too. Could devs do PM work, I think so, at least little bits!